EP0240583B1

EP0240583B1 - Heat storage composition

Info

Publication number: EP0240583B1
Application number: EP86104778A
Authority: EP
Inventors: Naomichi Yano; Tadatsugu Ueno; Shigeru Tsuboi
Original assignee: Kubota Corp
Current assignee: Kubota Corp
Priority date: 1986-04-08
Filing date: 1986-04-08
Publication date: 1991-07-03
Anticipated expiration: 2006-04-08
Also published as: AU587243B2; EP0240583A1; AU5576986A; DE3680101D1; EP0377473A3; EP0377473A2; US4715978A

Description

This invention relates to a heat storage composition for use in greenhouses for facility horticulture or cultivation, in living area heating, in chemical heat pumps, further in solar energy storage tanks and industrial waste heat recovery facilities, and in other fields; furthermore the invention relates to the use of said composition in latent heat storage capsules.
Calcium chloride hexahydrate has a solidification point of about 30°C, which is close to the ordinary temperature range, with a great latent heat of solidification/melting which is characteristic of a hydrate, and therefore is coming into wide and practical use in greenhouses for facility horticulture and plant cultivation, in living area heating, in chemical heat pumps, further in solar energy storage tanks and industrial waste heat utilization facilities, among others. However, this compound involves a serious problem that a marked supercooling phenomenon is observed with it. This is an obstacle to practical use of said compound. The phenomenon of supercooling is a phenomenon that the liquid-to-solid phase change does not begin in the process of cooling of a substance in the liquid phase even after passage of the solidification point but at last begins at a temperature considerably below the solidification point. When supercooling takes place, the solidification point at which the latent heat of solidification should be released becomes unspecified and this is a fatal defect in the use as a heat storage material for maintaining a specific temperature range. For solving such problem, a technique of preventing supercooling has been proposed (e.g. Japanese Patent Publication No. 32749/80 and No. 9059/81) which comprises adding to calcium chloride hexahydrate a nucleating agent capable of promoting crystallization thereof. Said technique is under development for early practical use. Many substances are known as nucleating agents for such use, for example, strontium chloride hexahydrate, strontium hydroxide octahydrate, strontium oxide, barium hydroxide octahydrate, barium carbonate and barium nitrate. Addition of these in an amount of 0.1-20 percent by weight on the whole heat-storage composition basis can prevent the supercooling of calcium chloride hexahydrate to a considerable extent.
However, check experiments made by the present inventors for evaluating the effects of various nucleating agents have revealed that any nucleating agent cannot prevent the occurrence of supercooling by about 3-4°C. Moreover, addition of more than 20 percent by weight of a nucleating agent cannot be expected to produce any further effect.
On the other hand, when calcium chloride hexahydrate is used alone, the latent heat release temperature is specifically restricted to one single point, namely about 30°C which is the solidification point (and at the same time the melting point) thereof, so that it is difficult to adjust the same to the use conditions with respect to said temperature. Therefore, the latent heat release temperature is generally adjusted by addition of a solidification point adjusting agent such as FeCl₃·6H₂O, MgCl₂·6H₂O or CoCl₂·6H₂O. However, the nucleation-promoting agents and solidification point modifiers, when used alone in heat-storage compositions, gradually lose their effects upon repeated use as a result of precipitation thereof in the heat-storage material-containing vessels and eventually their effects cannot be fully produced any more in some instances. It is also known that upon repeated liquid-solid phase changes, calcium chloride hexahydrate itself gradually precipitates on the vessel bottom due to a specific gravity difference between the liquid phase (having a specific gravity of 1.5) and the solid phase (having a specific gravity of 1.68), leading to phase separation.
Therefore, for the purpose of increasing the dispersion stability of additives including nucleation-promoting agents and preventing phase separation, a thickening agent is added to heat-storage compositions. The thickening agent is used to achieve the above purpose by providing a melt under use with an appropriate viscosity and includes, among others, alcohols, such as glycerin and ethylene glycol, carboxymethylcellulose and poly(sodium acrylate).
Among the above thickening agents, glycerin is particularly valuable since it is miscible with water in any proportion, is capable of providing an adequate viscosity and has good stability. However, since said substance has solidification point depressing activity, great variations in solidification point are inevitable even when it is used for the purpose of viscosity increase, particularly when it is used in relatively large amounts so as to attain high viscosity values. On the other hand, the use of those thickeners which are so far in general use, for example high-molecular substances such as poly(sodium acrylate) is disadvantageous in that although they have excellent viscosity increasing effects, repeated use thereof results in local caking and viscosity reduction and eventually in failure in its duty to produce homogeneous dispersion.
Failure in dispersion of the nucleating agent and other auxiliary ingredients leads to substantial failure in answering the intended purpose of their incorporation, namely loss of their ability to prevent the phenomenon of supercooling on the occasion of phase transition, and at the same time allows phase separation, whereby the value of the heat-storage material containing them is reduced.
Meanwhile, the use of latent heat-storage capsules with a latent heat-storage material capable of thermal phase change, namely a phase-change material, sealed therein (hereinafter, "PCM capsules") as heat sources for various purposes has been proposed, for example for storing solar energy therein for later heat radiation for heating purposes or, more broadly, for storing solar energy in summer for emission in winter for various heating purposes. Such PCM capsules are under way for practical use.
As the above-mentioned PCM capsules, there are known spherical ones (e.g. Japanese Utility Model Application No. 109283/83) and flat ones (e.g. Japanese Utility Model Application No. 105796/84), among others. From the viewpoints of ease in placing, ease in forced circulation of a heat transfer medium in heat exchange, and so forth, the latter flat PCM capsules may be said to be more advantageous.
In particular, for heat exchange between PCM capsules and air as a heat transfer medium, flat PCM capsules are preferable.
However, flat PCM capsules are very small in thickness as compared with the other dimensions, length and breadth, so that when they are in the vertical disposition, the latent heat-storage material, for example crystalline calcium chloride (CaCl₂·6H₂O), or a nucleating agent therefor contained in the flat PCM capsules precipitates on the container bottom, whereupon the crystal growth owing to the nucleating agent, namely the phase change of the latent heat-storage material, cannot be promoted in a uniform manner any more, hence, disadvantageously, the heat-storage effect cannot be produced to a satisfactory extent.
It is conceivable that horizontal disposition of flat PCM capsules might solve such problem.
In that case, the nucleating agent is dispersed uniformly and generally over the flat bottom portion of the flat PCM capsules and this favorably causes uniform phase change in the latent heat-storage material. However, when the temperature of the flat PCM capsules is lower than that of air and thus there is a temperature difference from the air in the stage of heat storing, dew condensation can easily occur on the flat PCM capsule surface. The water resulting from this dew condensation can hardly be discharged and moreover that portion of heat which is consumed for the vaporization of this water is directly reflected in a disadvantageously reduced heat-storage efficiency.
Furthermore, in using PCM capsules in temperature control apparatus for use in various hothouses and the like, it is necessary to provide a separate heating unit in addition to the PCM capsules so that the shortage of heat as resulting from insufficient heating, for example in winter when the duration of sunshine is short, can be filled up. When such a heating unit is used combinedly, heat radiation from said unit can hardly extend over the whole hothouse and this readily results in lack of uniformity in temperature within the hothouse. For avoiding such trouble, a blower is required for circulating the air within the hothouse to thereby cause the heat radiated extend over the whole hothouse.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings,
Fig. 1 graphically represents an example of the supercooling curve for the heat storage composition;
Figs. 2-11 each is a graphical representation of the degree of supercooling in a heat storage composition according to the invention;
Fig. 12 is a graphical representation of the relationship between the level of addition of a solidification point modifier and the solidification points;
Fig. 13 is a graphical representation of the relationship between the level of addition of a solidification point modifier and the quantity of heat stored;
Fig. 14 is a graphical representation of the relationship between the level of addition of an ultrafine silica powder or glycerin and the viscosity of the heat storage composition in the molten state;
Fig. 15 is a graphical representation of the influence of the level of addition of an ultrafine silica powder and of glycerin on the viscosity of the heat storage composition in the molten state;
Fig. 16 is a graphical representation of the relationship between the number of melting-solidification cycles and the degree of supercooling for an example of the heat storage composition according to the invention;

DETAILED DESCRIPTION OF THE INVENTION

An object of the invention, which has been worked out to solve the problems involved in the prior art as mentioned above, is to provide a heat storage composition which consists mainly of calcium chloride hexahydrate and is capable of substantially avoiding the phenomenon of supercooling and absorbing or releasing the latent heat of solidification with certainty at a temperature around the theoretical solidification point. Another object of the invention is to provide a heat storage composition which is highly stable with respect to phase separation among the elements constituting the heat storage composition, i.e. main constituent (calcium chloride hexahydrate), nucleating agent (barium sulfide, etc.), solidification point modifier (zinc chloride, etc.) and so on, and can produce a high-level heat storage effect even in repeated use thereof.
Such objects of the invention have been accomplished by providing the constitutions specified in the accompanying claims.
When a heat storage composition consisting substantially of calcium chloride hexahydrate alone is cooled from the molten state, it does not begin to solidify even after passage across its solidification point (about 29.5°C) but begins to solidify rapidly at about 20°C, for instance, as indicated by the solid line in Fig. 1. The degree of such supercooling varies greatly depending on the rate of cooling and the extent of disturbance of the melt, among others, so that the temperature at which the latent heat is released cannot be specified. Accordingly, the temperature control in response to a desired temperature cannot but become imprecise. When a nucleating agent for preventing supercooling, for example strontium chloride hexahydrate, is added to the composition in an amount of about 5 percent by weight, the phenomenon of supercooling is much inhibited and the degree of supercooling is reduced to about 3-4°C, as indicated by the broken line in Fig. 1. However, such supercooling inhibiting effect of known nucleating agents cannot be said to be fully satisfactory although the optional addition level differs only to some extent depending on the kind of the nucleating agent. Thus, it is not that supercooling can be controlled substantially within an acceptable range.
After a number of experiments with various compounds, the present inventors confirmed that the phenomenon of supercooling can be suppressed very effectively by using barium sulfide and barium chloride dihydrate combinedly in certain specific amounts. It was further found that, as will be described later in the examples, the coexistence, in a heat storage composition containing calcium chloride hexahydrate as the main component, of 0.001-5 percent of barium sulfide and 0.05-5 percent of barium chloride dihydrate can suppress the supercooling to at most 2°C. When the amount of barium sulfide or barium chloride dihydrate is lower than the lowest limit given above, the synergistic supercooling inhibiting effect arising from their combined use cannot be expected any more but only an incomplete supercooling inhibiting effect (supercooling of about 5-6°C) as obtainable by their single use can be produced. On the other hand, when the contents of the above two additives exceed the respective upper limits, solidification does not occur in some instances or the quantity of latent heat decreases greatly, so that the performance and stability of the heat storage material deteriorate.
In a further study, if was found that when a small amount of strontium chloride is used in combination with barium sulfide and barium chloride dihydrate, a satisfactory supercooling inhibiting effect can be secured even at a further reduced total nucleating agent addition level. In view of such excellent supercooling inhibiting effect of strontium chloride hexahydrate, it was expected that a satisfactory supercooling inhibiting effect might be still obtained even when one of barium chloride or barium sulfide is omitted, and investigations were conducted in this direction in an attempt to omit the use of barium sulfide which can be a source of hydrogen sulfide. As a result, it was found that a satisfactory supercooling inhibiting effect can be produced when strontium chloride hexahydrate and a slightly increased amount of barium chloride are used combinedly. After determination of the optimum contents of the above components, the present invention has now been completed.
Thus, in accordance with the invention, the contents of barium sulfide and the nucleating agents can be reduced to 0.0001-5 percent and 0.001-5 percent, respectively by adding 0.001-0.1 percent of strontium chloride hexahydrate as an additional nucleating agent to the whole heat storage composition, as will be detailedly described later in the examples. When strontium chloride hexahydrate is used in an amount of not less than 0.06 percent, the combined use of barium chloride dihydrate alone as another nucleating agent in an amount of not less than 0.5 percent can produce a satisfactory supercooling inhibiting effect. The supercooling inhibiting effect is dependable and sufficient at very low nucleating agent addition levels if the levels of addition of the nucleating agents meet the conditions given below.
Thus, the nucleating agent contents (or addition levels), X (%) for barium sulfide, Y (%) for barium chloride dihydrate and Z (%) for strontium chloride hexahydrate, are as follows:
0 ≦ X ≦ 5
0.001 ≦ Y ≦ 5,
0.001 ≦ Z ≦ 0.1, and

[I] when 0.06 ≦ Z ≦ 0.1, then
X = 0 and Y ≧ 0.5,
[II] when 0.005 ≦ Z ≦ 0.06, then
X ≧ 0.0001 and Y ≧ 0.01, or
[III] when 0.001 ≦ Z ≦ 0.005, then
X ≧ 0.001 and Y ≧ 0.01.

As mentioned above, a heat storage composition which will cause substantially no supercooling phenomenon and has an optionally selected latent heat release temperature can be obtained by incorporating into a heat storage material mainly consisting of calcium chloride hexahydrate a specific nucleating agent consisting of barium chloride and so on and further, optionally, a solidification point modifier, such as zinc chloride, potassium bromide, sodium bromide or ammonium bromide. Upon repeated use, namely after repetition of the solidification-melting cycle, even this heat storage composition may sometimes deteriorate in its performance as a result of precipitation of part of said nucleating agent or solidification point modifier as crystals. In such case, however, the dispersion stability of the whole heat storage composition can be markedly improved by incorporating into the heat storage composition an adequate amount of an ultrafine silica powder plus glycerin as a thickening agent. As said ultrafine silica powder, there may be used a high purity ultrafine silica powder, such as Aerosil (trademark) of Degussa, West Germany. Supposedly, such substance exhibits its thixotropic property owing to the action of the silanol group (≡Si-OH) which said substance has in its structure. Said substance occurs as very minute particles (7-40 µm) and is highly dispersible in various media. Thus, when incorporated into the heat storage composition, said substance is dispersed uniformly while maintaining the fine particulate state. It is presumable that, upon melting of said composition, particles of said substance are connected with one another by forming crosslinks and that, as a result, a thickening effect is produced.
Ultrafine silica powders have so far been used as thickening agents for paints or as sagging or running inhibitors for paints for thick coating of walls, among others, and their thickening effect is well known. Hithertofore, however, there have been no instances of their use as thickening agents for heat storage compositions.
The present inventors have confirmed that ultrafine silica powders produce excellent thickening effect in heat storage compositions which are in the molten state and are very stable both chemically and physically and little susceptible to different heat storage compositions or to environmental conditions, such as heat. Thus, addition in relatively small amounts of an ultrafine silica powder as a thickening agent together with glycerin to a heat storage composition whose main component is an inorganic substance in a hydrate form and which may optionally contain a solidification point modifier and/or a nucleation promoting agent gives a necessary and sufficient viscosity. Moreover, an ultrafine silica powder does not aggregate or cake or otherwise degrade even after repetition of the heat storage-release cycle. Furthermore, the addition of glycerin does not affect the solidification point since a low level of addition of glycerin is already sufficient. Therefore, the heat storage composition with an ultrafine silica powder and glycerin incorporated therein as thickening agents exhibits excellent repetition stability, reveals no ununiform dispersion or phase separation phenomenon, and can maintain a high level of dispersion stability for a prolonged period of time.
The following examples of the heat storage composition which is the most fundamental constituent element in the present invention, together with background experimental data which are the bases for establishing the relevant parameters, illustrate the invention in more detail.

EXAMPLES

Experiment series

1

The supercooling inhibiting effects of barium sulfide and barium chloride dihydrate each added alone as a nucleating agent to calcium chloride hexahydrate, as shown Tables 1 and 2, are shown in Fig. 2 and Fig. 3, respectively. In the experiments, 0.001-10 percent of barium sulfide or barium chloride dihydrate was added to calcium chloride hexahydrate and each heat storage composition was tested for the degree of supercooling (cf. Fig. 1) by repeating the melting-solidification cycle.
As is apparent from Fig. 2 and Fig. 3, the single use of BaS or BaCl₂·2H₂O cannot produce a satisfactory supercooling inhibiting effect. In the above experiments, the nucleating agent contents in composition No. 6 and No. 11 were too large, namely these compositions did not solidify at their respective proper solidification points at all, hence could not be used as heat storage compositions.
In the next place, the supercooling inhibiting effect of the combined use of barium sulfide and barium chloride dihydrate each in an appropriate amount was examined. Thus, as shown in Table 3, heat storage compositions were prepared in which the contents of barium sulfide and barium chloride dihydrate were varied, and they were tested for the degree of supercooling by repeating the melting-solidification cycle.
The results obtained are shown in Figs. 4-8. In the figures, the numbers correspond to the experiment numbers given in Table 3.
The results of these experiments suggest:

(1) That when the barium sulfide addition level is less than 0.001 percent, the supercooling inhibiting effect is not sufficient even when the barium chloride dihydrate addition level is in a proper range and that, conversely, when the barium sulfide addition level exceeds 5 percent, solidification may not take place in some instances (Experiment No. 21, No. 26, No. 31 and No. 36) and, even if solidification occurs, the latent heat of solidification becomes reduced and the performance of the relevant composition as a heat storage material becomes markedly decreased.
(2) That when the barium chloride dehydrate addition level is less than 0.05 percent, any synergistic supercooling inhibiting effect cannot be produced in its combined use with barium sulfide, the degree of supercooling always exceeding 2-3°C. That when the barium chloride dihydrate content exceeds 5 percent, solidification does not occur in some cases like in the case of excessive barium sulfide content. It was further confirmed that even when solidification occurs, the latent heat of solidification becomes markedly small.
(3) That, on the contrary, the use of barium sulfide and barium chloride dihydrate each in an adequate amount results in synergistic increase in their supercooling effect and, as a result, the degree of supercooling can be limited to at most 2°C in any case.

Now, the results of experiments which serve to confirm the effect of strontium chloride hexahydrate as the nucleating agent are described.
Heat storage compositions in which the content of strontium chloride hexahydrate was varied as shown in Table 4 were prepared and examined for the supercooling inhibiting effect.
As seen from the results shown in Fig. 9, for securing a satisfactory supercooling effect, strontium chloride hexahydrate must be used in an amount of not less than 0.1 percent.

Example 1

Based on the above experimental results, it was considered that when strontium chloride hexahydrate is used combinedly with the above barium sulfide and/or barium chloride dihydrate, the content of each of these nucleating agents might be further reduced. Accordingly, the supercooling inhibiting effect was studied for cases in which these three were used combinedly. Thus, heat storage compositions in which the contents of the above three nucleating agents were varied each in a lower addition level range, as shown in Table 5, were prepared and examined for the supercooling inhibiting effect.
The results obtained are shown in Fig. 10 and Fig. 11. As is evident from Fig. 10, the combined use of the above three nucleating agents can reduce to a significant extent the addition levels or contents of the respective agents as required for securing the desired supercooling inhibiting effect as compared with the single use thereof or the combined use of two of them. As Fig. 10 indicates, it is advisable that when the amount of strontium chloride hexahydrate is rather small (0.001-0.05 percent), the amount of barium sulfide should be increased to some extent (not less than 0.001 percent). When strontium chloride dihydrate is used in a relatively large amount (0.01-0.05 percent), a satisfactory supercooling effect can be obtained even at a relatively low barium sulfide addition level (not less than 0.0001 percent). Furthermore, the data shown in Fig. 11 indicate that when strontium chloride hexahydrate is used in a relatively large amount (0.06-0.1 percent) and barium chloride dihydrate is used combinedly in an amount of not less than 0.5 percent, a satisfactory supercooling inhibiting effect can be obtained even in the absence of barium sulfide. After all, an excellent supercooling inhibiting effect can be obtained by using the three nucleating agents in small amounts and adjusting their addition levels such that the above conditions [I], [II] and [III] are satisfied.
Whereas the synergistic supercooling inhibiting effect producible by barium sulfide, barium chloride dihydrate and strontium chloride hexahydrate in heat storage compositions whose main component is calcium chloride hexahydrate is as above mentioned, it is usual in practical use of heat storage compositions to further use a thickening agent and/or a solidification point modifier combinedly. Therefore, several typical examples of the heat storage composition which contain these additive components are given below, together with the solidification point and the degree of supercooling (mean of 10 repeated cycles) for each composition. As regards the solidification point modifier, detailed mention will be made later in describing a further experiment series.

Experiment series 2

Fig. 12 is a graphic representation of the tendency toward depression of the solidification point of a heat storage composition whose main component is calcium chloride hexahydrate and which contains as a solidification point modifier 5-50 percent of ferric chloride hexahydrate, calcium bromide hexahydrate, potassium bromide, sodium bromide or ammonium bromide. As is evident from this figure, it is possible to adjust the solidification point as desired within the range of about 30°C and about 15°C by using potassium bromide, sodium bromide or ammonium bromide, selected as a preferred solidification point modifier according to the invention, at an addition level lower than the addition levels for the conventional solidification point modifiers (e.g. ferric chloride hexahydrate, calcium bromide hexahydrate). As mentioned earlier herein, an increasing amount of a solidification point modifier shows a tendency toward decrease in the quantity of latent heat of the heat storage composition itself, while the quantity of latent heat in the use temperature range is essentially required to be large for a composition to be an excellent heat storage composition. However, to lower the solidification point of a eutectic mixture directly leads to a decreased potential energy of the eutectic mixture, so that, essentially, a decrease in the quantity of latent heat cannot be avoided. Thus, the essential problem is by what means the decrease in the quantity of latent heat which accompanies solidification point depression should be minimized. The above-mentioned bromides are smaller in the quantity of latent heat as compared with the conventional solidification point modifiers, hence can serve to adjust the solidification point as desired without causing significant deterioration in the performance of the heat storage composition. Fig. 13 shows the change in the quantity of latent heat in a heat storage composition, whose main component is calcium chloride hexahydrate, with a varying amount of each of the above three bromides as added to said composition, in comparison with theoretical values calculated on the basis of the heat of fusion for calcium chloride hexahydrate (45.6 cal/g) and with a conventional modifier (zinc chloride). As is evident from Fig. 13, when the above bromides are incorporated, the latent heat values differ little from the theoretical values at various addition levels. On the contrary, in the case of the conventional modifier (zinc chloride), the tendency toward decrease in the quantity of latent heat as compared with the theoretical values is remarkable and, when compared at an equal addition level, the latent heat is much less as compared with the bromides. Moreover, the differences therebetween increases with the increase in the addition level. What has been mentioned above may be summarized in Table 6. Table 6 shows the addition levels required to adjust the solidification point to 20°C and the latent heat quantities at said solidification point for the above bromides and some typical conventional modifiers (ferric chloride hexahydrate, magnesium chloride hexahydrate and cobalt chloride hexahydrate). As is evident from Table 6, potassium bromide, sodium bromide and ammonium bromide can give the desired solidification point in about one third of the addition levels required for the conventional solidification point modifiers and the latent heat quantities at said temperature for the bromides are 1.5- to 2-fold larger as compared with the conventional compositions. Thus, the use of at least one of potassium bromide, sodium bromide and ammonium bromide in accordance with the invention can give a heat storage composition having an optionally chosen solidification point with a high level of latent heat quantity.

Experiment series 3

Fig. 14 shows the thickening effects produced by addition of an ultrafine silica powder and glycerin to a basic heat storage composition (1) given in Table 7.
The solid line in the figure is for the case in which the ultrafine silica powder alone was added as the thickening agent, and the broken line is for the case in which glycerin was added alone. As is evident from Fig. 14, the ultrafine silica powder has excellent thickening effect. It gave high viscosity values in lower concentrations as compared with glycerin, in particular at addition levels not lower than 3.5 percent. However, the ultrafine silica powder showed a rapid viscosity increase after the addition level exceeds 3.5 percent. This means that a small difference in addition level means a great variation in viscosity. Such situation is unfavorable from the viscosity adjustment viewpoint and makes it difficult to specify a desired viscosity particularly in the manufacture of heat storage compositions.

Example 2

Fig. 15 shows the data obtained by adding, to the basic heat storage composition (1) given in Table 7, an ultrafine silica powder alone (solid line), the ultrafine silica powder and 1 percent of glycerin combinedly (dot-and-dash line), the ultrafine silica powder and 3 percent of glycerin combinedly (dot-dot-dash line) and the ultrafine silica powder and 5 percent of glycerin (broken line), respectively. Whereas, as mentioned above, the single use of the ultrafine silica powder at an addition level of about 3.5 percent or above results in a rapid viscosity increase, so that fine viscosity adjustment is practically difficult in said range, Fig. 15 reveals that the use of the ultrafine silica powder in combination with glycerin makes gentle the ultrafine silica powder content-viscosity curve and furthermore gives an adequate viscosity increase curve also in the addition level range below 3.5 percent. Thus, the ultrafine silica powder and glycerin cooperate in a complementary manner across the boundary at the ultrafine silica powder addition level of about 3.5 percent to give a gentle and adequate viscosity increase curve as a whole. While the pattern of the viscosity curve for the combined use of these two thickening agents is affected in a complicated manner by the addition levels for the respective additives including both the thickeners and other factors, it is advisable and preferable for adjusting the viscosity of the heat storage composition to add glycerin in an amount of 1-5 percent and the ultrafine silica powder in an amount of 1.5-6 percent. By suitably adjusting the proportion between both the thickening agents and the total addition level therefor, it is possible to obtain, in heat storage compositions, any desired viscosity within a broad range stably.

Example 3

Fig. 16 shows the stability of a heat storage composition specified in Table 8 as composed of calcium chloride hexahydrate as the main component, a solidification point modifier (zinc chloride) and nucleating agents (barium chloride dihydrate, barium sulfide and strontium chloride hexahydrate) after addition of (1) an ultrafine silica powder and glycerin as thickening agents each in an amount of 3 percent (solid line), (2) glycerin in an amount of 5 percent (dot-and-dash line) as a thickening agent, or (3) without addition of any thickening agent. The stability data obtained by repeating the melting-solidification cycle are shown in the figure for comparison.
As is evident from Fig. 16, the composition after addition of 3 percent each of the ultrafine silica powder and glycerin retained a degree of supercooling as low as about 1.5°C even after 300 times of repeated use, and the increase in the degree of supercooling as observed after continued repeated use was always slight. Even after 700 or more times of repeated use, the solidification point depression remained not more than 2.5°C. On the contrary, without the thickening agents, the degree of supercooling showed a tendency toward rapid increase from the beginning of repeated use and, after about 100 times of use, said degree reached a level as high as 4.8°C. The composition for comparison (dot-and-dash line) containing 5 percent of glycerin alone retained a degree of supercooling not greater than 2°C approximately at the 250th cycle. This degree of supercooling, or performance, was comparable to that found in the case of the combined use of the ultrafine silica powder and glycerin [composition (1)]. However, after the 250th cycle, the degree of supercooling increased as a result of gradual phase separation. Thus it can be understood that the single use of glycerin along as the thickening agent gives only compositions lacking in long-term stability in repeated use thereof.
The following are typical examples of the heat storage composition which contain an ultrafine silica powder and glycerin as thickening agents and characteristics thereof.
The present invention has the above-mentioned constitution and the effects of the invention may be summarized as follows:

(1) The phenomenon of supercooling in heat storage compositions whose main component is calcium chloride hexahydrate can be radically reduced or substantially prevented by combinedly using barium sulfide, barium chloride dihydrate and strontium chloride hexahydrate as nucleating agents each in a small amount in said compositions. Therefore, the temperature at which the latent heat is utilized can be controlled exactly and precisely without any substantial decrease in heat storage capacity.
(2) The melt viscosity of such heat storage composition as mentioned above can be adjusted as desired within a relatively broad range by combinedly using an ultrafine silica powder and glycerin as thickening agents each in a small amount. The resulting composition does not deteriorate with respect to its performance characteristics upon repeated use thereof.

Claims

A heat storage composition containing calcium chloride hexahydrate as the main component, which contains, as nucleating agents for preventing supercooling, 0-5 percent by weight (on the whole heat storage composition basis) of barium sulfide, 0.001-5 percent by weight (on the same basis) of barium chloride dihydrate and 0.001-0.1 percent by weight (on the same basis) of strontium chloride hexahydrate, wherein the barium sulfide content (X percent), barium chloride dihydrate content (Y percent) and strontium chloride hexahydrate content (Z percent) further satisfy the following conditions:
when 0.06 ≦ Z ≦ 0.1, then
X = 0 and Y ≧ 0.5,
when 0.005 ≦ Z ≦ 0.06, then
X ≧ 0.0001 and Y ≧ 0.01, or
when 0.001 ≦ Z ≦ 0.005, then
X ≧ 0.001 and Y ≧ 0.01.
The heat storage composition of Claim 1 or 2, which further contains at least one bromide selected from the group consisting of potassium bromide sodium bromide and ammonium bromide as a solidification point modifier.
The heat storage composition of Claim 1 or 2, which additionally contains a ultrafine silica powder and glycerin as thickening agents.
The heat storage composition of Claim 3, wherein the ultrafine silica powder is present in an amount of 1.5-6 percent by weight and glycerin in an amount of 1-5 percent by weight.
A latent heat storage capsule containing the heat storage composition of any one of claims 1 to 4.
Use of the heat storage composition of any one of claims 1 to 4 in a latent heat storage capsule.